The present invention relates to a tire condition monitoring apparatus that permits a driver in a vehicle passenger compartment to check the conditions of tires, such as the air pressure, and to a tire condition monitoring apparatus. More particularly, the present invention relates to a tire condition monitoring apparatus having a transponder that wirelessly transmits data representing the condition of a tire in response to an interrogating radio wave, and a transmitter-receiver that transmits the interrogating radio wave and receives the data wirelessly transmitted by the transponder.
Conventionally, tires having a transponder are used. Such a transponder has a coil antenna for wirelessly transmitting the condition of a tire attached to a vehicle. When receiving an outside interrogating radio wave, an electric power is induced by the coil antenna. Based on the induced electric power, the transponder wirelessly transmits data representing, for example, the condition and identification of the tire (Japanese Laid-Open Patent Publication No. 5-169931).
In the art disclosed in the above-mentioned publication, however, the time intervals at which the interrogating radio wave is transmitted from the outside are constant regardless of the speed of the vehicle. Correspondingly, the transponder responding to the interrogating radio wave wirelessly transmits data including tire identification data always at constant time intervals. Therefore, even in a case where there is a need to immediately notify a driver of a tire abnormality, for example, during high-speed traveling, the notice is sent at the same time intervals as at the time of stoppage or low-speed traveling.
An antenna for transmitting the interrogating radio wave is provided in the vicinity of the corresponding transponder, e.g., in a wheel well. On the other hand, the tire in which the transponder is provided rotates as the vehicle travels. When the tire rotates, the transponder also rotates. Consequently, the distance between the antenna and the transponder is relatively changed. The wheel made of a metal exists between the antenna and the transponder particularly when the antenna and the transponder are in such positions that the distance therebetween is maximized. This means that there is a possibility of the transponder failing to normally receive the interrogating radio wave. In such a case, electric power necessary for enabling the transponder to operate is not generated in the coil antenna. As a result, the transponder cannot operate normally and transmit data representing the condition of the tire.